Mr. Gould remarked to me long
ago, that in those genera of birds which range over the world, many of
the species have very wide ranges. I can hardly doubt that this rule
is generally true, though difficult of proof. Among mammals, we see it
strikingly displayed in Bats, and in a lesser degree in the Felidae and
Canidae. We see the same rule in the distribution of butterflies and
beetles. So it is with most of the inhabitants of fresh water, for many
of the genera in the most distinct classes range over the world, and
many of the species have enormous ranges. It is not meant that all, but
that some of the species have very wide ranges in the genera which range
very widely. Nor is it meant that the species in such genera have, on an
average, a very wide range; for this will largely depend on how far the
process of modification has gone; for instance, two varieties of the
same species inhabit America and Europe, and thus the species has an
immense range; but, if variation were to be carried a little further,
the two varieties would be ranked as distinct species, and their range
would be greatly reduced. Still less is it meant, that species which
have the capacity of crossing barriers and ranging widely, as in the
case of certain powerfully-winged birds, will necessarily range widely;
for we should never forget that to range widely implies not only the
power of crossing barriers, but the more important power of being
victorious in distant lands in the struggle for life with foreign
associates. But according to the view that all the species of a genus,
though distributed to the most remote points of the world, are descended
from a single progenitor, we ought to find, and I believe as a general
rule we do find, that some at least of the species range very widely.

We should bear in mind that many genera in all classes are of ancient
origin, and the species in this case will have had ample time for
dispersal and subsequent modification. There is also reason to believe,
from geological evidence, that within each great class the lower
organisms change at a slower rate than the higher; consequently they
will have had a better chance of ranging widely and of still retaining
the same specific character. This fact, together with that of the seeds
and eggs of most lowly organised forms being very minute and better
fitted for distant transportal, probably accounts for a law which has
long been observed, and which has lately been discu